Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in an LNG plant at or near the source of a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressure. Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through at least one cooling stage using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas can then be further expanded to final atmospheric pressure suitable for storage and transportation.
The at least one cooling stage can comprise pre-cooling and main cooling stages, which sequentially reduce the temperature of the natural gas. The main cooling stage may be carried out in at least one main heat exchanger, to provide a liquefied, partially or fully liquefied, hydrocarbon stream, such as LNG.
U.S. Pat. No. 6,272,882 discloses a process for liquefying a gaseous, methane-rich feed stream to obtain LNG. The process utilises two cooling stages, a propane pre-cooling refrigerant cycle and a mixed refrigerant main cooling cycle. A main heat exchanger defining a shell side within its walls and at least one tube side extending through the shell side is used to liquefy natural gas in the main cooling stage. The natural gas is passed through one of the tube sides in hydrocarbon stream flow tubes where it is indirectly cooled and liquefied against the mixed main refrigerant in the shell side of the heat exchanger.
U.S. Pat. No. 6,272,882 employs advanced process control strategies, utilising mass flow rates of main refrigerant fractions and the hydrocarbon stream to be cooled, amongst others, as manipulated variables and the temperature differences within the main heat exchanger, amongst others, as controlled variables in order to optimise the production of LNG.
The advanced process control method of U.S. Pat. No. 6,272,882 can lead to changes in the mass flow rate of the hydrocarbon stream to be cooled as a manipulated variable.
In addition to changes in mass flow of the hydrocarbon stream as a result of advanced process control methods, a reduction in this mass flow may occur as a result of the partial shutdown of the liquefaction facility for repair and maintenance (so-called “turn down operation”), or during periods of lower demand for LNG.
A reduction in the mass flow of the hydrocarbon stream from the designed operational conditions can result in a decrease in the frictional pressure drop of the hydrocarbon stream across the main heat exchanger(s), increasing the potential for unstable behaviour in the cooling process.